IEC 13522-3-00:2004 – Information Technology – Coding of Multimedia and Hypermedia Information – Part 3: MHEG Script Interchange Representation

Scope and Applicability of IEC 13522-3-00

IEC 13522-3-00 (2004), formally part of the ISO/IEC 13522 family of standards, defines the representation of scripts within the Multimedia and Hypermedia information coding Expert Group (MHEG) framework. This part, commonly referred to as MHEG-3, specifies the syntax and semantics of an interchange format for scripts – procedural program fragments that extend the dynamic behavior of MHEG objects.

The standard applies to systems where multimedia and hypermedia applications need to exchange active content logic independently of the underlying platform or programming language. It enables script code to be transmitted, stored, and instantiated across heterogeneous environments. The scope covers:

• A formal notation for script constructs using Abstract Syntax Notation One (ASN.1), as defined in ISO/IEC 8824.
• Transfer syntax encodings – both binary and textual – for script interchanges, optimized for low-bandwidth and real-time constraints.
• Binding with MHEG-1 objects (scene description and objects) to allow scripts to reference and manipulate hypermedia elements.
• Support for multiple script language profiles, with the default profile based on the MHEG Script language (a subset of JavaScript-like syntax with MHEG-specific extensions).

IEC 13522-3-00 is critical for systems that require dynamic content adaptation, such as interactive digital television, networked kiosks, and tele-learning platforms, where scripts manage navigation, event handling, and data updates.

Technical Requirements: MHEG Script Encoding and Interchange

ASN.1 Module Definition

The core of IEC 13522-3-00 is its ASN.1 module that defines the abstract syntax of MHEG scripts. The module includes types such as ScriptObject, ScriptTerminal, StatementSequence, and Expression. Each script object is encapsulated as an MHEG-1 compatible Object with a ScriptClass tag. The standard mandates that all scripts shall be syntactically correct according to this ASN.1 specification.

Encoding Techniques

Two encoding rules are defined:

• Binary Encoding (BER): Optimized for size, using a tag-length-value structure. It reduces overhead for embedded systems and broadcast environments.
• Textual Encoding (XER): Human-readable XML encoding that facilitates debugging and authoring tools. It is required for interoperability in web-based MHEG implementations.

The encoding rules ensure that a script can be decoded unambiguously regardless of the source platform, with mandatory support of both encodings for a conforming decoder.

Binding to MHEG-1 Objects

Scripts operate on MHEG-1 objects (e.g., BooleanVariable, Bitmap, AudioSegment) through a well-defined API. The script interpreter must provide access to the runtime environment, including object state, event queues, and timing services. The standard specifies a set of intrinsic functions (e.g., GetValue, SetValue, AddListener) that all implementations must support.

Key Technical Specifications

Implementation Highlights and System Integration

Decoder Architecture

A typical implementation of IEC 13522-3-00 involves a Script Decoder that parses the incoming byte stream, builds an abstract syntax tree (AST) using the ASN.1 definitions, and then executes the script within a sandboxed interpreter. The interpreter must manage memory efficiently, as scripts can be loaded incrementally.

Authoring Considerations

Content creators should follow these best practices:

• Use XER encoding for authoring and testing, then switch to BER for deployment to reduce size.
• Bind scripts to MHEG-1 objects using the provided API rather than directly manipulating object internals, to maintain forward compatibility.
• Validate script syntax with a conformance checker that tests against the ASN.1 module and the language profile.

Interoperability with Other MHEG Parts

The standard is designed to work seamlessly with MHEG-1 (ISO/IEC 13522-1) and MHEG-5 (ISO/IEC 13522-5). Scripts can be embedded within Scene objects or referenced as external resources. The runtime integration points include:

• Event injection: Scripts can trigger MHEG events (UserInput, TimerExpired) which are then processed by the MHEG engine.
• Object mutation: Scripts can alter MHEG object properties (e.g., visibility, position, content URL) by calling SetValue on the object’s state variables.
• Streaming synchronization: For continuous media, scripts can call SyncOn to align with MPEG-2 or MPEG-4 timelines.

Compliance, Testing, and Certification Notes

Conformance Requirements

To claim conformance with IEC 13522-3-00, an implementation must satisfy the following mandatory conditions:

• Decoder conformance: Must correctly decode BER and XER encodings for all script structures defined in the ASN.1 module.
• Interpreter conformance: Must support all intrinsic functions and the language profile (MHEG Script) without divergence in semantics.
• Runtime conformance: Must enforce the allowed operations and security restrictions (e.g., no access to file system unless explicitly allowed).

Testing Methods

The ISO/IEC JTC 1/SC 29 committee provides a set of conformance test suites (e.g., ISO/IEC 13522-3:2000/Amd 1:2003) that include:

• Positive test scripts covering valid object constructions, arithmetic operations, and event flows.
• Negative tests for error handling, edge cases (e.g., zero-length scripts, deeply nested loops).
• Encoding round-trip tests to verify that BER → ASN.1 → XER conversions are lossless.

Certification Bodies

Industry certification for MHEG products is typically offered by organizations such as the DVB Project (for digital TV profiles) and national testing centers (in Europe and Asia). Products must pass an MHEG-3 conformance test suite to receive a compliance mark. The standard also includes provisions for declaring conformance Class A or B, which should be stated in the product documentation.